The present invention relates to the manufacture of integrated circuits. More particularly, the present invention relates to wafer stain monitoring apparatus and methods therefor.
In the manufacture of semiconductor integrated circuits, the wafer may at times be processed using wet chemicals. For example, some process requires the appropriately patterned wafer to be immersed in wet, liquid chemicals for etching. After the exposed areas of wafer are etched by the wet chemicals, the wafer is then removed, washed with deionized water, and dried.
After washing and drying, it has been observed that silica stains sometimes appear on the surface of the wafer. The silica stains are caused by dissolved silica in the washing solution, which is redeposited on the surface of the wafer during drying. If a sufficiently thick layer of silica stain is left behind after washing and drying the wafer, the stain may affect subsequent processes. By way of example, the unwanted silica stains represent contaminants on the wafer, which may affect the electrical performance of the IC after fabrication. As another example, the silica stain may act as an unintended mask in a subsequent etch step. As can be appreciated by those skilled in the art, such silica stains are generally undesirable.
To monitor for the presence and amount of silica stain on the wafer surface, a silica test structure is usually needed. The silica stain test structure is designed to enhance the formation of silica stains, thereby rendering them more susceptible to detection and monitoring.
In the prior art, various silica stain test structures have been proposed. To facilitate discussion, FIG. 1 illustrates a prior art planar silicon stain test structure 100, which is typically formed atop a silicon wafer 102. Above silicon wafer 102, there is shown a plurality of silicon dioxide (SiO.sub.2) strips 104. Silicon dioxide strips 104 are typically formed by first blanket depositing a layer of silicon dioxide above silicon wafer 102 and then etching back some of the silicon dioxide (using an appropriate photoresist mask and etchant) to form the strip pattern.
As the wafer containing silica stain test structure 100 is washed and dried, the alternating strips of silicon and silicon dioxide materials permit the silica-containing droplets of the wash solution to adhere to the surface of the test structure. When the droplets dry, the stain remains. The stain is depicted in FIG. 2 as stain 202 on silica stain test structure 100.
In FIG. 2, stain 202 is depicted as being fairly visible and hence detectable. In reality, prior art silica stain test structures (such as prior art silica stain test structure 100 of FIGS. 1 and 2) typically do not yield stains of sufficient sizes and thicknesses to be detectable by conventional (e.g., 10X-100X) optical microscopes. In the typical situation, it is necessary in the prior art to perform additional processing on the stains in order to facilitate stain detection and measurement. By way of example, one prior art stain monitoring technique requires that the stains formed on the prior art silica stain test structure be further etched via reactive ion etching (RIE) before it can be detected and/or measured. The presence and amount of stain may be ascertained by, for example, monitoring the RIE distortion.
As can be appreciated by those skilled in the art, the additional processing steps required to detect and/or measure stains formed on prior art silica monitor test structure are highly disadvantageous. At minimum, the additional processing represents an inconvenience to the process engineers, who must perform additional work before the presence and/or amount of silica stain can be detected. Additional processing also involves additional complexities, which can often be measured in terms of additional required time and/or expenses.
In view of the foregoing, there are desired improved silica stain monitoring techniques.